1. Field of the Invention
The present invention relates to a chuck used to hold a substrate during a lithographic process. More specifically, this invention relates to a process for bonding silicon silicon carbide to glass ceramics to form a chuck.
2. Background
Lithography is a process used to create features on the surface of substrates. During lithography, a wafer is disposed on a wafer stage and held in place by a chuck. The chuck is typically a vacuum chuck capable of securely holding the wafer in place. The wafer is exposed to an image projected onto its surface by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each can require a different exposure apparatus, as is known to those skilled in the relevant art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the sharpness of the image projected onto the wafer is maximized. While using a step-and-scan technique generally assists in improving overall image sharpness, image distortions generally occur in such systems due to imperfections within the projection optics system, illumination system, the particular reticle being used and temperature differentials between the wafer and lithography tools.
Lithography tools typically require a thermally stable environment. Light sources and illumination during lithography introduces heat into the system. A typical temperature range for a scanning lithography tool would be between about 18 and 22° C. While under steady state scanning conditions, temperatures in a wafer chuck itself may vary plus-or-minus about one (+/−1)° C. Nonetheless, because of the extremely small tolerances of the precision stages (e.g., on the order of about 10 nm or less), even small changes in temperature can cause unintended thermally induced changes in the dimensions of precision stages. Therefore, methods for controlling temperature in lithography tools, and in particular wafer chucks, during lithography scanning is required.
Lithography tools also require extremely quiet environments with respect to motion and vibration. For this reason, chucks are typically magnetically positioned and propelled within lithography tools. This places great demands on scanning and alignment control systems. The extent of control is directly related to system frequency, which is, in turn, directly related to the specific stiffness of the chuck. Therefore, what is needed are lithography tools, including wafer chucks, with improved properties that allow for temperature control and high quality lithographic scanning.